1. Field of the Invention
The present invention relates to a droplet ejection device for ejecting droplets toward a receiver medium.
2. Discussion of Related Art
There is known an inkjet printer which is capable of printing desired patterns of image on a printing medium, by ejecting ink droplets subjected to a gradation control, toward the printing medium through nozzles arranged in a print head of the printer. As the print head of such an inkjet printer, there is known a print head which is equipped with piezoelectric elements adapted to pressurize an ink, for ejecting the pressurized ink in the form of ink droplets. Owing to its accurate controllability and excellent durability, such a print head is widely used. The print head, required to perform a printing operation on a receiver medium with high resolution, is provided by a serial head repeatedly reciprocated above the receiver medium, or provided by a line head having nozzles a number of which corresponds to a required degree of resolution. Where the print head is provided by the serial head, there is a problem that the printing operation can not be performed at a high speed since the print head has to be reciprocated many times. Where the print head is provided by the line head, there is a problem that the print head can not be manufactured at a low cost since a large number of nozzles are necessarily arranged on the print head.
In view of such problems, there is proposed an inkjet printer, as disclosed in U.S. Pat. No. 4,468,679 (equivalent to JP-A-S57-185159), which includes a pair of nozzles each controlled to control timing and velocity of ejection of an ink droplet from the corresponding nozzle, such that the droplets ejected from one and the other of the pair of nozzles collide with each other before landing on a receiver medium. The colliding droplets form a united droplet which is placed at a desired position (see FIG. 2 of the U.S. publication).
In the inkjet printer disclosed in the above-identified U.S. publication, it is possible to place the droplet at a selected one of a large number of different positions with a small number of nozzles provided in the print head. Owing to this advantage, where the print head is provided by a serial head, the printing speed can be increased with a reduced number of times of reciprocating motions of the print head. Where the print head is provided by a line head, the manufacturing cost can be reduced with a reduced number of nozzles to be provided in the print head. In the inkjet printer, however, the timing and velocity of the ejection of each ink droplet have to be controlled with considerably high accuracy requiring a complicated control system. Further, even if the complicated control system is established in the printer, the ejection control can not be achieved with high degree of freedom.